Humans are able to hear across a wide range of sound levels, both in quiet and in the presence of background noise. The auditory system may achieve this wide dynamic range by turning down its response when there is ongoing sound. Though there is physiological evidence for a feedback loop to the cochlea which adjusts gain in a frequency-specific manner. Temporal effects have been measured in simultaneous masking which would be consistent with a decrease in gain with sound stimulation. It is not clear whether these temporal effects are due to the processing of the feedback system. The proposed research examines this important question in light of more recent evidence about the peripheral auditory system. The experiments are designed so that two measures of gain in the cochlea, input-output functions and frequency selectivity, may be estimated with and without preceding acoustic stimulation designed to activate the efferent system. Listeners with normal hearing and listeners with mild cochlear hearing loss will be tested in the experiments. The hearing-impaired listeners provide an interesting control case in which the gain in some frequency channels has been permanently decreased. The data will be analyzed within a well-established physiologically-based model of the peripheral auditory system, to determine whether temporal effects are consistent with a decrease in gain within the auditory channel which is stimulated. This research is important to our understanding of basic auditory processing in normal hearing, and the effects of cochlear hearing impairment. Listeners with normal hearing may have a type of automatic gain control that works independently in each of the thousands of auditory channels in the cochlea. This gain control could help in the perception of speech in background noise, by improving the signal-to-noise ratio within a channel, and by turning down channels containing only noise. The loss of this mechanism could help explain why listeners with cochlear hearing impairment have particular problems in background noise. This research could suggest novel processing strategies that might be used in hearing aids and cochlear implants to improve perception in background noise.The proposed research examines the hypothesis that listeners with normal hearing may have a type of automatic gain control that works independently in each of the thousands of auditory channels in the cochlea. This gain control could help in the perception of speech in background noise, by improving the signal-to-noise ratio within a channel, and by turning down channels containing only noise. The loss of this mechanism could help explain why listeners with cochlear hearing impairment have particular problems in background noise. This research could suggest novel processing strategies that might be used in hearing aids and cochlear implants to improve perception in background noise. [unreadable] [unreadable] [unreadable]